The effect of ethanol on human sensorimotor reactivity was assessed by examining the acoustic startle response. Twelve healthy normal subjects participated in a startle reflex experiment in which placebo or ethanol were given on separate days. Three types of startle probes were used. They consisted of pulse-alone bursts of white noise at 108 dB(A) and 99 dB(A) to explore startle reactivity, and of a 108 dB(A) pulse preceded by a 85 dB(A) prepulse stimulus (prepulse+pulse) to assess prepulse inhibition. Startle amplitude was larger to the 108 dB(A), compared to the 99 dB(A) pulse-alone probes. The prepulse stimulus significantly reduced the amplitude of the startle reflex elicited by the subsequent 108 dB(A) stimulus. The amplitude of the startle response was dramatically reduced by acute ethanol. The effects of ethanol on prepulse inhibition could not be assessed because the startle response was too small in the ethanol condition.